Most ‘home compostable’ plastics do not break down in garden bins

Compostable plastic that has not fully decomposed in a compost bin

Compostable plastic that has not fully decomposed in a compost bin

www.bigcompostexperiment​ (CC BY)

Most compostable plastics labeled as suitable for home composting do not break down properly in garden bins, leaving plastic residue that pollutes garden soils and even enters the food chain.

The findings prompted researchers to call for an overhaul of certification standards and led at least one company to phase out the use of compostable plastics altogether.

More than 1600 members of the public took part in the Big Compost Experiment, a UK-wide study to test how well compostable plastic packaging breaks down in household compost bins.

They composted packaging such as newspaper and magazine wrappers, food wrappers and shopping bags for three to 12 months, then sifted through the resulting plastic to identify any remaining plastic.

Most of the objects were judged to still be clearly visible to the naked eye, often in large intact pieces. Only about a third of items were reported to have been fully composted, while about 60 percent of certified “home compostable” plastics did not break down properly.

Danielle Purkiss at University College London (UCL) says the results expose a major problem with “home composting” certifications.

These include controlled laboratory experiments using a fixed type of compost, a specific set of microorganisms and small samples of compostable materials, he says.

However, home composting comes in all shapes and sizes, with conditions varying depending on the type of compost bin used, the types of soil in the garden or acreage, and where the composters live.

“We’re showing that some of the underlying standards and tests to prove the performance of these materials — they don’t actually reflect the real world they’re going into,” he says. “And that’s a real red flag for us – it means that, in fact, the standards and the certification are not fit for purpose.”

Entry into the food chain

Purkiss says that compost produced in gardens and allotments across the UK is used to grow food, vegetables and culinary herbs and that there is therefore a high risk of plastic waste entering the food chain. “We know there’s a pathway into the soil, and therefore into the food chain, and into things that we eat or other organisms,” he says.

Compostable plastics have grown in popularity as brands seek more sustainable alternatives to oil-based plastic packaging.

But most compostable plastics end up being burned or sent to landfill in the UK because there is no dedicated collection route to take the waste to industrial composters.

Some people put compostable packaging in food waste collections, but it is usually treated as a pollutant and fished out.

Home composting is therefore one of the only reliable ways for customers to ensure that their compostable plastic is actually composted.

But Purkiss says packaging producers should now think twice before instructing the public to compost such plastic at home.

“You shouldn’t use the term ‘home composting’ if it doesn’t represent the true variety of environments that comprise home composting,” he says.

Phasing out of compostables

This week, organic food company Abel & Cole said it was phasing out compostable plastics in its packaging in light of evidence from UCL.

Hugo Lynch, the company’s sustainability project manager, says the findings from the study dovetail with feedback from Abel & Cole customers, a “significant number” of whom wrote to complain that packaging was not being properly composted in its bins their home.

“There are a number of products that are certified [degrade] within a set time frame,” he says. “However, based on our experience, the experience of our customers, the discussions we’ve had independently with waste processors and also the results of the UCL reports, it’s clear that a lot of the things that are certified under these conditions – just don’t happen in real conditions”.

Lynch says Abel & Cole has now switched to using alternative papers where possible and is working with suppliers to ensure compostable plastic is completely phased out of its range by the end of next year.

There is a risk that the debate about home compostables will be blown out of proportion, says David Newman of the Bio-based and Biodegradable Industries Association (BBIA), which represents producers of biodegradable and compostable packaging.

Newman describes home composting as a “backyard hobby activity” that many people aren’t very good at. “It’s a very small part of the population that actually does it, and an even smaller part of the population that does it right,” he says. “To say home composters don’t work means your composting doesn’t work either.”

A better solution, he says, would be to allow people to put compostable materials in their food waste bin. Most councils allow food waste bins to be included, but will fish out any other compostable plastic wrap. Routing compostable packaging to industrial composting, an industry it says is “run by professionals”, would ensure the products are fully broken down. “The moral of the story is that we need to sort out our food waste collection systems,” he says.

Journal Reference: Limits to sustainabilityDOI: 10.3389/frsus.2022.942724

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